Abstract: A device for the measurement of the wall thickness of tubes, in particular for the measurement of extruded synthetic tubes during the extrusion process, comprises a common support for location within the tube and a plurality of measuring bodies mounted on the support and urged resiliently outwards for engagement with the tube wall. In order to provide a sufficiently robust support for the measuring bodies they are connected to the support by means of a parallelogram linkage, and in order to allow the measuring bodies a degree of free movement so that they can conform to the contour of the tube wall at least one parallelogram arm of the parallelogram linkage is pivoted with free play.

Abstract: Apparatus for measuring the wall thickness of a moving tube, for example an extruded tube of synthetic plastics material, comprises means for location in a stationary position within the tube and adapted to present at least one measuring body in resilient contact with the inner wall of the tube, and an external sensor for sensing the position of the measuring body. A mechanical linkage incorporating means engaging the tube wall is arranged to lift the measuring body away from the tube wall when excessive friction between the tube wall and the engagement means indicates a condition liable to damage the measuring body.